Impurities contained in a semiconductor wafer have a large influence on the performance of a semiconductor device. For this reason, it is desirable to know and control an impurity concentration during manufacture of a semiconductor wafer. For example, carbon in a semiconductor wafer lowers the withstand voltage of a power device. Accordingly, demands have risen for reducing a carbon concentration in, e.g., a single-crystal silicon layer formed on an epitaxial silicon wafer by an epitaxial growth method (referred to as an epitaxial silicon layer, hereinafter).
A silicon wafer having a single-layer structure obtained by slicing an ingot normally contains carbon at a relatively high concentration (1×1015 atoms/cm3 or more), and this carbon concentration can be measured by a known method. Also, since this silicon wafer has the single-layer structure, the carbon concentration can be measured by a transmission method such as Fourier-transform infrared spectroscopy (FT-IR).
On the other hand, in many epitaxial silicon wafers, an epitaxial silicon layer has a carbon concentration of less than 1×1014 atoms/cm3. This concentration is lower than the lower detection limit of an apparatus which measures carbon concentration by known methods such as secondary ion mass spectrometry (referred to as SIMS, hereinafter). Also, since the epitaxial wafer has a multilayer structure, when the transmission method is used, layers other than the epitaxial silicon layer are also measured, and so transmission methods such as infrared absorption spectrometry cannot be applied.
Jpn. Pat. Appln. KOKAI Publication No. 2013-152977 describes a method of measuring the concentration of an impurity in a semiconductor wafer by a photoluminescence spectroscopy (referred to as a PL method, hereinafter). In this method, a calibration curve indicating the relationship between the concentration of an impurity in a semiconductor wafer and the intensity of photoluminescence (referred to as PL, hereinafter) deriving from the impurity is formed in advance. Then, the PL intensity of a semiconductor wafer having an unknown impurity concentration is measured, and the impurity concentration is obtained by referring the measurement result to the calibration curve. Note that the calibration curve is formed by using a semiconductor wafer having a known impurity concentration.
As described above, in many epitaxial silicon wafers, the carbon concentration of an epitaxial silicon layer is lower than the lower detection limit of a measurement apparatus. Therefore, when the method described in Jpn. Pat. Appln. KOKAI Publication No. 2013-152977 is applied to the measurement of the carbon concentration in the epitaxial layer, it becomes difficult to form a calibration curve having a small error.